Patent Document 1 describes a method for manufacturing a single-crystal gallium nitride crystal at a uniform epitaxial growth rate, and this crystal has good cleavage and low dislocation density. According to this method, after numerous isolated shielding portions are formed on a substrate of a different crystal, gallium nitride is grown by vapor-phase deposition into a sufficiently thick gallium nitride crystal with closed defect-concentrated regions H extending over the shielding portions. Subsequently, a free-standing gallium nitride frame substrate with a thickness of 100 to 500 μm is prepared by removing the back substrate and polishing the gallium nitride crystal or by cutting the crystal parallel to its surface. The closed defect-concentrated regions are then removed from the gallium nitride frame substrate by dry etching with hydrogen chloride gas, thus obtaining a gallium nitride skeletal substrate including only single-crystal low-dislocation-density accompanying regions and a single-crystal low-dislocation-density remaining region. A less-defect-concentrated gallium nitride crystal without closed defect-concentrated regions is manufactured by growing gallium nitride on the gallium nitride skeletal substrate by vapor-phase deposition.
Patent Document 2 describes a method for manufacturing a single-crystal substrate. According to this method, a crystal including both a portion with polarity A and a portion with polarity B is used. The portion with polarity B is completely or partially removed by etching to form a base, and a crystal is grown thereon again so that a crystal with polarity A is formed over the surface of the base or the entire single crystal with polarity A is formed thereon. Alternatively, the portion with polarity B is covered with a different material after being partially removed or not being removed, and the same crystal is grown again so that the surface is covered with a crystal with polarity A. The single-crystal substrate manufactured by this method has a surface of a single crystal with polarity A which is suitable for formation of an electronic device thereon.
Patent Document 3 describes a method for growing single-crystal gallium nitride and for manufacturing a single-crystal gallium nitride substrate. According to this method, a mask with a regularly striped pattern is provided on a base substrate, and linear V-grooves (troughs) defined by facets are formed thereover. Gallium nitride is facet-grown while the V-grooves are maintained so that defect-concentrated regions H are formed on the bottoms of the V-grooves (troughs) defined by the facets. Consequently, dislocations are concentrated to form the defect-concentrated regions H and therefore results in fewer dislocations in the surrounding regions, namely, low-dislocation-density single-crystal regions Z and c-plane growth regions Y. The defect-concentrated regions H trap and do not release the dislocations that have trapped because the regions H are closed. Facet growth, in which gallium nitride is grown while facets are formed and maintained, has a disadvantage of forming dislocations extending from the centers of pits defined by facets; they form planar defects extending radially. In addition, a device cannot be provided thereon because the positions where pits are formed cannot be controlled. These can be alleviated by the method described in Patent Document 3.
Patent Document 4 describes a method for growing single-crystal gallium nitride and for fabricating single-crystal gallium nitride. According to this method, a regular seed pattern is provided on a base substrate, and gallium nitride is facet-grown thereon while pits defined by facets are formed and maintained so that closed defect-concentrated regions H are formed on the bottoms of the pits defined by the facets. As a result, dislocations are concentrated to form the defect-concentrated regions H, while the dislocation density of the surrounding regions, namely, single-crystal low-dislocation-density accompanying regions Z and a single-crystal low-dislocation-density remaining region Y, is reduced. The closed defect-concentrated regions H trap and do not release the dislocations because the regions H are closed.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2004-59363
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2004-221480
Patent Document 3: Japanese Unexamined Patent Application Publication No. 2003-183100
Patent Document 4: Japanese Unexamined Patent Application Publication No. 2003-165799